Nucleopolyhedrosis viruses appear not to be involved in diseases other than those of insects. Because of a narrow host specificity they are attractive alternatives to chemical insecticides and will probably be used in increasing amount, it is important that the characteristics of the virus infection be studied at the molecular level. The major problems to be considered are: 1) host range of the virus; 2) latency of the virus; 3) means for monitoring the presene of the viral genome; and 4) the developement of methods for detecting changes in viral genetic material during commercial production of the viral insecticides. Some of the problems have been examined using cytopathogenic effect to determine viral effects. However, this means of analysis is not satisfactory for long-term evaluation of viral insecticides. We are investigating two nucleopolyhedrosis viruses that infect the Douglas-fir tussock moth by examining viral DNA replication and expression of the DNA in the host cell. Specifically, we will examine the transcription of polyhedrin mRNA, the primary structure of polyhedrin, and the synthesis of viral and cellular DNA in infected cells. We shall examine latency and host range of NPBV using the presence of viral DNA and the synthesis of viral proteins to detect viral components in the challenged cell. We shall establish restriction endonuclease fragment patterns characteristic of the virions that will be useful for detecting genetic change in the viruses. We will use the tussock moth NPBV and NPSV in these studies because these virions are active components of the viral insecticide TM Biocontrol-1 which was developed to control the Douglas-fir tussock moth infestations.